Dielectric Laser Research Articles

Power oscillations induced by the relative Goos-Hänchen phase

By using an optical interferometer composed of a dielectric laser ellipsometer, to change the optical response of transverse electric and magnetic incident radiation, and two polarisers, to trigger the interference pattern induced by the relative Goos-Haenchen phase, we show under which conditions it is possible to optimize the laser power oscillations induced by the relative phase difference between orthogonal polarised states. The Goos-Haenchen interference can then be used to sense rotation, to test optical components, and to simulate quarter and half wave plates.

Investigation of dielectric layers laser ablation mechanism on n-PERT silicon solar cells for (Ni) plating process: Laser impact on surface morphology, composition, electrical properties and metallization quality

Laser contact opening is a critical step for solar cells manufacturing and needs to be optimized to achieve high efficiencies. In this paper, laser contact opening using a picosecond laser (wavelength 355 nm, pulse duration 10 ps) has been carried out on n-PERT precursors composed of a SiOx/SiOxNy stack on the rear polished side and a SiOxNy layer on the front textured side. By varying peak fluence from 0.130 J/cm2 to 2.159 J/cm2 and spot overlapping, ninety parameters combinations have been tested to open these dielectric layers. Surface morphology characterization, before and after laser ablation, has been realized using Confocal Laser Scanning Microscopy and Scanning Electron Microscopy. Bulk and surface compositions have also been investigated by Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy analysis, respectively. Results have shown the existence of four separated laser impacted areas on the polished side and a related ablation mechanism is suggested. Also, electrical characterization using four probe measurements and calibrated lifetime photoluminescence revealed that electrical properties of the silicon underlying increased when post laser annealing was performed associated with no spot overlapping. Then, nickel electroless deposition has been performed and first characterizations indicate adherence issues and inhomogeneous metallization. Characterization of metallized samples revealed that these observations were closely linked to the non-homogenous surface morphology and composition after laser ablation.

Reconfigurable Photonic Circuit for Controlled Power Delivery to Laser-Driven Accelerators on a Chip

Dielectric laser acceleration (DLA) represents a promising approach to building miniature particle accelerators on a chip. However, similar to conventional RF accelerators, an automatic and reconfigurable control mechanism is needed to scale DLA technology towards high energy gains and practical applications. We present a system providing control of the laser coupling to DLA using integrated optics and introduce a novel component for power distribution using a reconfigurable mesh of Mach-Zehnder interferometers. We show how such a mesh may be sequentially and efficiently tuned to optimize power distribution in the circuit and find that this strategy has favorable scaling properties with respect to size of the mesh.

Anomalous Photon-Induced Near-Field Electron Microscopy.

We reveal the classical and quantum regimes of free electron interaction with radiation, common to the general variety of radiation sources (e.g., a Smith-Purcell radiation), the dielectric laser accelerator, and photo-induced near-field electron microscopy (PINEM). Modeling the electron with initial conditions of a coherent quantum electron wave packet, its topology in phase space uniquely defines a universal distinction of three interaction regimes: point-particle-like acceleration, a quantum wave function (PINEM), and a newly reported regime of anomalous PINEM (APINEM). The quantum interference beat of APINEM is capable of improving the spectral resolution of postselective electron microscopy. The particle-wave duality transition between regimes reveals the history-dependent nature of quantum electron interaction with light.

Dielectric laser electron acceleration in a dual pillar grating with a distributed Bragg reflector.

We report on the efficacy of a novel design for dielectric laser accelerators by adding a distributed Bragg reflector (DBR) to a dual pillar grating accelerating structure. This mimics a double-sided laser illumination, resulting in an enhanced longitudinal electric field while reducing the deflecting transverse effects when compared to single-sided illumination. We improve the coupling efficiency of the incident electric field into the accelerating mode by 57%. The 12μm long structures accelerate sub-relativistic 28keV electrons with gradients of up to 200 MeV/m in theory and 133 MeV/m in practice. This Letter shows how lithographically produced nano-structures help to make novel laser accelerators more efficient.

Silicon nitride waveguide as a power delivery component for on-chip dielectric laser accelerators.

We study the weakly guided silicon nitride waveguide as an on-chip power delivery solution for dielectric laser accelerators (DLAs). We focus on the two main limiting factors on the waveguide network for DLAs: the optical damage and nonlinear characteristics. The typical delivered fluence at the onset of optical damage is measured to be ∼0.19 J/cm2 at a 2μm central wavelength and 250fs pulse width. This damage fluence is lower than that of the bulk Si3N4 (∼0.65 J/cm2), but higher than that of bulk silicon (∼0.17 J/cm2). We also report the nonlinearity-induced spectrum and phase variance of the output pulse at this pulse duration. We find that a total waveguide length within 3mm is sufficient to avoid significant self-phase modulation effects when operating slightly below the damage threshold. We also estimate that one SiNx waveguide can power 70μm silicon dual pillar DLAs from a single side, based on the results from the recent free-space DLA experiment.

A Numerical Investigation of Dielectric Laser Accelerators for Nuclear Security-Related Applications Using LSP

This paper investigates the potential use of dielectric laser accelerators (DLAs) for nuclear security applications. For the first time, the LSP code is used to look at this application. LSP is a fast running, parallelized particle-in-cell code that includes space charge effects. The focus of this paper is to ascertain the order of magnitude, amount of charge, that can be driven through the accelerator. From these results, a recommendation on the potential of DLAs for nuclear security applications is given.

Enhanced energy gain in a dielectric laser accelerator using a tilted pulse front laser.

Using an 800 nm, 45 fs pulse-front-tilted laser we demonstrate a record 315 keV energy gain in a dual grating dielectric laser accelerator (DLA) and average accelerating gradients of 560 MV/m over 0.5 mm. These results open a new regime in DLA characterized by significant evolution of the beam distribution in the longitudinal phase space, corresponding to > 1/4 of a synchrotron oscillation. By tilting the laser wavefront we control the resonant velocity of the DLA and observe a net energy gain, indicating that a tapered optical phase could be used to achieve very high energy gain.

Grating-based waveguides for dielectric laser acceleration

We propose a chip-scale hollow-core waveguide using high-contrast gratings as reflectors for dielectric laser acceleration. We show that confinement of a specified accelerating mode can be achieved by adjusting the thickness of a matching layer between the core and the highly reflective grating. Several examples of the grating-based waveguide and their characteristic parameters such as the group velocity, interaction impedance, and acceleration efficiency are presented. The planar structure of the waveguide makes fabrication and integration simple, which is required by an on-chip dielectric laser accelerator for high-energy, material, and medical applications.

Beam dynamics studies for beam focusing and solenoid alignment at SINBAD

SINBAD facility (Short INnovative Bunches and Accelerators at DESY) under construction at DESY plans to host several experiments for the production of ultra-short bunches and will be a test facility for high-gradient compact novel acceleration techniques. The ARES (Accelerator Research Experiment at SINBAD) linac is foreseen to produce ultra-short bunches to be injected e.g. into Novel Dielectric Laser Acceleration structures or Laser Wake-Field Acceleration experiments. The work presented in this paper is based on optimization of the focusing system consisting of solenoids for the ARES, which have been studied earlier in detail but is revisited for updated beamline. Moreover tolerances for the possible misalignment of solenoids are presented investigating the effect on the beam properties during the gun commissioning.

Simulation of phase-dependent transverse focusing in dielectric laser accelerator based lattices

The Accelerator on a CHip International Program (ACHIP) funded by the Gordon and Betty Moore Foundation aims to demonstrate a prototype of a fully integrated accelerator on a microchip based on laser-driven dielectric structures until 2021. Such an accelerator on a chip needs all components known from classical accelerators. This includes an electron source, accelerating structures and transverse focusing arrangements. Since the period of the accelerating field is connected to the drive laser wavelength of typically a few microns, not only longitudinal but also transverse effects are strongly phase-dependent even for few femtosecond long bunches. If both the accelerating and focusing elements are DLA-based, this needs to be taken into account. In this work we study in detail the implications of a phase-dependent focusing lattice on the evolution of the transverse phase space of a transported bunch.

First order sensitivity analysis of electron acceleration in dual grating type dielectric laser accelerator structures

Symmetrically driven dual-grating type DLA (Dielectric Laser Accelerator) linac structures allow for in-channel electric field gradients on the order of GV/m at optical wavelengths. In this work we study the sensitivity of important final beam parameters like mean energy, energy spread and transverse emittance on DLA drive laser as well as input beam parameters. To this end a fast specialized particle tracking code (DLATracker) is used to compute the so called first order sensitivity indices based on a large number of Monte Carlo simulation runs of an exemplary external injection based DLA experiment. The results of this work point out important stability constraints on the drive laser setup and the externally injected electron beam.

Design of a tapered slot waveguide dielectric laser accelerator for sub-relativistic electrons.

We propose a dielectric laser accelerator design based on a tapered slot waveguide structure for sub-relativistic electron acceleration. This tapering scheme allows for straightforward tuning of the phase velocity of the accelerating field along the propagation direction, which is necessary for maintaining synchronization with electrons as their velocities increase. Furthermore, the non-resonant nature of this design allows for better tolerance to experimental errors. We also introduce a method to design this continuously tapered structure based on the eikonal approximation, and give a working example based on realistic experimental parameters.

High-field nonlinear optical response and phase control in a dielectric laser accelerator

Advances in ultrafast laser technology and nanofabrication have enabled a new class of particle accelerator based upon miniaturized laser-driven photonic structures. However, developing a useful accelerator based on this approach requires control of the particle dynamics at field intensities approaching the damage limit. We measure acceleration in a fused silica dielectric laser accelerator driven by fields of up to 9 GV m−1 and observe a record 1.8 GV m−1 in the accelerating mode. At these intensities the dielectric is driven beyond its linear response and self-phase modulation changes the phase velocity of the accelerating mode, reducing the average gradient to 850 MeV m−1. We show that free-space optics can be used to compensate this dephasing and demonstrate that tailoring the laser phase and amplitude can facilitate optimization of the beam dynamics. This could enable MeV scale energy gain in a single stage and pave the way towards applications in scientific, industrial, and medical fields.

Computational screening of organic polymer dielectrics for novel accelerator technologies

The use of infrared lasers to power accelerating dielectric structures is a developing area of research. Within this technology, the choice of the dielectric material forming the accelerating structures, such as the photonic band gap (PBG) structures, is dictated by a range of interrelated factors including their dielectric and optical properties, amenability to photo-polymerization, thermochemical stability and other target performance metrics of the particle accelerator. In this direction, electronic structure theory aided computational screening and design of dielectric materials can play a key role in identifying potential candidate materials with the targeted functionalities to guide experimental synthetic efforts. In an attempt to systematically understand the role of chemistry in controlling the electronic structure and dielectric properties of organic polymeric materials, here we employ empirical screening and density functional theory (DFT) computations, as a part of our multi-step hierarchal screening strategy. Our DFT based analysis focused on the bandgap, dielectric permittivity, and frequency-dependent dielectric losses due to lattice absorption as key properties to down-select promising polymer motifs. In addition to the specific application of dielectric laser acceleration, the general methodology presented here is deemed to be valuable in the design of new insulators with an attractive combination of dielectric properties.

Optimized operation of dielectric laser accelerators: Multibunch

Optimized operation of dielectric laser accelerators: Multibunch

Simulation of phase-dependent transverse focusing in dielectric laser accelerator based lattices

Simulation of phase-dependent transverse focusing in dielectric laser accelerator based lattices

Optimization of electron bunch injection in dielectric laser acceleration

Dielectric laser acceleration (DLA) is new cheap and miniature laser acceleration scheme for acceleration of charged particles in the vicinity of the dielectric grating. In this paper injection characteristics of the electron bunch in a single fused silica grating based-structure with period λg = 260 nm is analyzed. Optimum aspect ratios of the grating width d/λg and height h/λg are obtained for the first surface mode harmonic. Simulation results of the injected electron dynamics show that there are two lateral positions (y = 1.75 and y = 2.6 μm) that the final electron energy is maximized. Existence of these two lateral positions increases flexibility of the injection process. In other words, injection can be made in the range of 1.4 < y(μm)<2.8 with two peaks of energy. Finally, injection of Gaussian electron bunch at each two optimum transverse positions is investigated by a numerical simulation. The initial momentum distribution is assumed to be Gaussian around the mean resonance velocity (β = λg/λ = 0.33). The initial longitudinal and transverse emittance of the electron bunch are selected as ε|| = (δE/E) = 0.01 and, ε⊥ = 3μmmrad respectively. The net energy gain in the fifty grating periods is 2.03 keV (2.225 keV) and acceleration gradient for each injection position is about 160 MeV/m.

Elements of a dielectric laser accelerator

We experimentally demonstrate several physical concepts necessary for the future development of dielectric laser accelerators—photonic elements that utilize the inelastic interaction between electrons and the optical near fields of laser-illuminated periodic nanostructures. To build a fully photonic accelerator, concatenation of elements, large energy gains, and beam steering elements are required. Staged acceleration is shown using two spatio-temporally separated interaction regions. Further, a chirped silicon grating is used to overcome the velocity dephasing of subrelativistic electrons with respect to its optical near fields, and last, a parabolic grating geometry serves for focusing of the electron beam.

On-Chip Laser-Power Delivery System for Dielectric Laser Accelerators

We propose an on-chip optical power delivery system for dielectric laser accelerators based on a fractal 'tree-branch' dielectric waveguide network. This system replaces experimentally demanding free-space manipulations of the driving laser beam with chip-integrated techniques based on precise nano-fabrication, enabling access to orders of magnitude increases in the interaction length and total energy gain for these miniature accelerators. Based on computational modeling, in the relativistic regime, our laser delivery system is estimated to provide 21 keV of energy gain over an acceleration length of 192 um with a single laser input, corresponding to a 108 MV/m acceleration gradient. The system may achieve 1 MeV of energy gain over a distance less than 1 cm by sequentially illuminating 49 identical structures. These findings are verified by detailed numerical simulation and modeling of the subcomponents and we provide a discussion of the main constraints, challenges, and relevant parameters in regards to on-chip laser coupling for dielectric laser accelerators.